Training for a marathon relay leg

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OK, I seem to have been persuaded to take part in my gym's relay team for the Belfast Marathon on 7 May.

So that's 5 weeks I have, to train up for it. The length will be between 3.5 - 7 miles, I dont know which I'll be doing so let's aim for the 7 mile one.

Whilst I'm generally fit, I'm not that experienced at running.

So, any tips for putting together a training shcedule?